Ultra high frequency transmitter



Original F'iled May 6, 1936 o E Dow E'mn. ULTRA HIGH FREQUENCYTRANSMITTER pr 8, 19M.

Patented Apr. 8, 1941 UNITED STATES PATENT OFFICE ULTRAHIGH FREQUENCYTRANSll/IITTER Orville E. Dow and Nils E. Lindenblad, Port J efferson,N. Y., assignors to Radio Corporation of America, a corporation ofDelaware (Cl. Z50-36) 6 Claims.

This application is adivision of our application Serial No. 78,124,filed May 6, 1936, now United States Patent No. 2,174,154, grantedSeptember 26, 1939.

This invention relates to ultra high frequency transmitters, and moreparticularly to a high quality television transmitter for efficientlytransmitting signal impulses whose frequencies cover a widev range fromapproximately twenty cycles to one and one-half million cycles. Suchfrequencies are known as video frequencies.

The complete transmitting circuit of the invention comprises, in brief,a push-pull master oscillator whose frequency is controlled by aconcentric line resonator, there being coupled to the output of theoscillator, a push-pull power amplier to whose grids arel coupled theamplified video frequencies of an extremely wide band. The oscillator isarranged to generate oscillations whose frequencies are of the order of177 mcgacycles corresponding to 1.695 meters. In the output of the poweramplifier and coupled to it through a novel type of Locher wiretransformerv is a monitor circuit for checking the. operation of thetransmitter. Among thel features of the invention are: The tunedconcentric line resonator Whose frequency is vari-able by changing atwill the capacity between the conductors, and the novel tuned circuitwhose uniformly distributed inductance is mostly shielded except for a`portion coupled to the power amplifier circuit. A complete description.of the invention follows, accompanied by a drawing, wherein the singlefigure shows schematically a complete transmitting system includingmaster oscillator, power amplifier, modulator and monitor circuits.

Referring to the drawing in more detail, there is. shown a masteroscillator circuit comprising a pair of electron discharge devices I, 2arranged in push-pull relation. There is provided a filament source ofsupply 3 connected in common to the filaments of both electron`discharge device oscillators'. Each filament has in series.- with eachofn its legs an. inductance 4 which is: of such;

value that, in cooperation. with the` tuning condenser 5 and seriesconnected by-pass condensers B- for radio frequencies, proper"regeneration is obtained in the oscillator'forbest efficiency. The

equivalent inductive reactance of 4 is variableby` the lcondenser 5v inshunt with it. A filament return. bias resistor I isz sho-wn connectedbctweenoneA leg of', the-filament and ground.

Connected to. the; grids., of theoscillator devices I, 2, is: a: tunedconcentric tubular conducto-r resonator line circuit comprising an innerconductor El` and an outer conductor IIJ for controlling the frequencyof the oscillations generated by the master oscillator. This frequencycontrol resonator is generally of the type described in the article byClarence W. Hansell, published in the A. I. E. E., August, 1935, pages852-857, except for certain features which will appear more fullyhereinafter. The grids of the electron discharge devices I, 2 areinductively coupled to the inner conductor 9 of the line by' means oftwo single-turn coils placed radially in4 the space between the innerand outer conductors and so connected as to impress radio frequencyvoltages of opposite phase on the grids of the vacuum tubes. In order toreduce thek inductive reactance of the grid circuits of. both vacuumtubes, so as to keep the regeneration of proper phase, there is provideda parallel shunt circuit 8. Inner conductor 9 of the tuned concentricline resonator is, in practice, made` to have a length slightly lessthan one-quarter of the length of the operating wave, and the capacitybetween` the top I I of the resonator and the top of the inner line 9varied to give the desired frequency. The inner conductorA comprises asa part thereof a pair of plates I2, I3 which are connected togetherconductively by a screw and a phosphor bronze spring I5. In oneposition, both plates I2 and I3 may be made to appear like a singleplate, in which case there will be minimum, capacity between the innerconductor 9 and the upper part II of outer conductor I0 with aconsequent maximum frequency of oscillation in the resonator. At theother extreme posi tion, by turning screw. I6 connected to shaft I4,both plates will provide a maximum capacity between the inner conductor9 and plate II with a consequent minimum frequency of oscillations inthe resonator.

The output of the master oscillator comprises a parallel tuned circuitconsisting ofr a variable condenser I'I and an linductance I3, I9', theinductor of which hasl substantially uniformly' distributed constants.of two copper rods I8 and I B' of equal length and enclosed byconcentric copper sleeves I9, I9 which are grounded. One conductor I8is' connected to the anode of tube 2 while conductor I8 is connected tothe anode of tube I. The opposite ends of I8 and, I' are connected.togetherby4 any suitable conductor i4 so positioned over ground, sheet29 as to make. its capacity' per unity length the same.- as theconcentric: condoctors Ill-t8. and ISL-ISI. The conductor lill must belong enough to provide sufficient cou- This: inductor is composed? plingto the power amplier grid circuit. In view of the physical configurationof inductance I8, I8', which includes two parallel connected portionsand a straight portion 44, it will herein be referred to as a Lecherwire output. The length of this Lecher wire output, as measured from theanodes of the electro-n discharge device I, 2 to the center point ofportion 44 of the Lecher wire system, is made to be about oneeighth of awavelength in order to prevent reaction upon the master oscillatordevice due to modulation of the power amplifier. The load on the masteroscillator, which in this case is the power amplifier grid circuit, maybe replaced by an equivalent series resistance (variable over lthemodulation cycle) in series with the Lecher wire output I8, I8' of thetwo master oscillator electron discharge devices. The inductivereactance of the output I8, IB at the operating frequency is fairly low,of the order of 100 ohms, and hence the equivalent series loadresistance may be made small, about two ohms. Consequently, anyvariation of the series load resistance of two ohms due to modulationwill have little effect on the master oscillator anode inductivereactance of 100 ohms.

Coupled to the straight portion 44 of the Lecher wire output I8, I8 area pair of power amplifier electron discharge devices 2l, 22 arranged inpush-pull relation to each other. The filament circuits of the tubes 2|,22 are heated by a common source of energy 23 which feeds the filamentsthrough radio frequency choke coils 24. Condensers 25 serve to by-passthe radio frequency energy in the filament connections to ground. A pairof neutralizing condensers 2l cross-connect the grids of the devices 2l,22 to the anodes of the associated devices in the conventional manner.The electron discharge devices, 2|, 22, are capable of oscillating atfrequencies as high as 400 megacycles per second, hence it is necessaryto make the leads from the grid and anodes to the neutralizingcondensers as short as possible. If these leads are made even moderatelylong, the relatively low inductance of the leads will have anappreciable inductive reactance at frequencies of the order of 300megacycles. This inductance may be sufciently high to allow the tubes tooscillate at some very high frequency. The ideal condition is to havezero reactance between the grid and neutralizing condenser and betweenthe plate and neutralizing condenser.

The center tape of the power amplifier grid circuit is connected througha resistance 4I to the anodes of the modulator tubes 40 which aremaintained at a positive potential of about 250 volts. However, thefilaments of the power amplifier devices 2|, 22 are maintained at apositive potential of 500 volts relative to ground, and consequently itcan be said that the grids of the power amplifiers are at an equivalentnegative potential of 250 volts relative to the filaments thereof. Thisarrangement of connecting the anodes of the modulation tubesconductively to the power amplifier filament return bias by 250 voltssimplifies the design of the circuit greatly.

The output circuit of the power amplifier comprises a symmetrically bentLecher wire system comprising two halves 28, 28 connected to which, inparallel, is a variable condenser 29 for tuning the output. The twohalves 28, 28' are provided merely to give symmetry to the physicalconfiguration ofthe circuit, so that in case" there is any influence onone half of the tuned circuit by the adjacent filament of internal tubeelements, there will also be a similar effect on the other half of theLecher wire output. Of course, it will be understood that, if desired,one-half of the Lecher wire output for the power amplifier can bedispensed with.

Coupled to the power amplifier output through leads 30 there are shown apair of terminals which connect with any suitable utilization circuitsuch as an antenna. It will be observed that leads 30 are inductivelyconnected to the Lecher wire system 28, 29 by means of a loop, thecenter point of which is grounded at 3l. This ground connection servesto prevent push-push .voltages on the antenna system. Also coupled toleads 3o is a step-down transformer comprising a U- shaped loop 32 Whoselegs are preferably each a quarter Wavelength long, as indicated,although the length of each leg can be any odd number of quarterwavelengths long. This loop is the equivalent of a step-down transformerand serves to step-down the impedance at the antenna terminals from ahigh value to the low value at the lower end of the loop, which is showninductively coupled to a diode 33. 'I'his U-shaped stepdown transformer,it will be observed, is also grounded at the center point 34 to preventpushpush oscillations. Loop 32 is merely a coupling link between theoutput of the power amplier and a monitor circuit comprising diode 33whose filament is closely coupled inductively to the stepdowntransformer 32. In the output circuit of diode 33, which in practice maycomprise an RCA type 955 tube, is a filter circuit 35, here shownconventionally, in turn coupled to a voltmeter 3G. Circuit 35 serves tofilter out the carrier frequency from the voltmeter. The purpose of themonitoring circuit is to measure the amplitude of the carrier from thepower amplifiers, and also to measure the modulation on the carrier. Itshould be noted that the quarter wavelength long step-down transformer32 presents a high impedance to the antenna terminals in order toprevent loading.

The modulator circuit shown within the box 31 serves to amplify thevideo frequency input from the value of .35 volt rms, as supplied fromthe studio to the first amplifier, to about 8G volts rms as effective inmodulating the grids of the electron discharge devices 2|, 22. For thispurpose there is provided a first pentode low power tube 38, which inpractice may be an RCA SCG, which drives a second pentode tube 39 whoseoutput is, in turn, connected to the grids of two other pentode tubes 48in parallel. From the output of pentode tubes 40 is a connection to thegrids of the power amplifier. In series with this connection is aresistor 4I to prevent push-push oscillations in the power amplifier.This resistor acts as a large series reactance to voltages of thecarrier frequency and has small stray capacitance for voltages of thevideo frequencies, and damps high frequency push-push parasiticoscillations. 'I'his resistor may, if desired, 'be omitted altogether,in which case the center point of the grid inductance will be directlyconnected to the modulator plates by a very ne (small diameter)resistance wire. This fine resistance wire acts as a large seriesreactance to voltagesl of the carrier frequency, and has small straycapacitance for voltages of the video frequency, thus damping receivedpush-push oscillations. Electron discharge devices 39 and 3G may, inpractice, lcomprise RCA type 802 tubes. All pentode tubes are of thewell-known type which include besides the anode and cathode, a controlgrid and a suppressor grid connected in known fashion. Coupled to theanodes of the two parallel connected Video modulators 4D is a loadresistor 42 which, in one embodiment tried satisfactorily in practice,had a value of 2250 ohms, and an inductance 43 which serves tocompensate for the tube and circuit output capacity. The videofrequencies supplied from the studio may have a very wide rangeextending from, let us say, 20 cycles to 1,500,000 cycles.

In the operation of the transmitter circuit, the voltage impulses fromthe master oscillator, which have a frequency of the order of 177megacycles per second, are impressed on the power amplier grids inpush-pull relation, and amplified through amplifiers 2|, 22. The degreeof amplification is varied by Varying the bias of the grids of 2l, 22 inparallel in accordance with voltage impulses which range in frequencyfrom 20 cycles per second to 1.5 megacycles per second, supplied by thetelevision pick-up mechanism and amplified by amplier circuit 3l. Theresulting amplitude modulated signal is sent out over the air throughthe antenna circuit coupled to the Lec-her wire output of the poweramplifier. The monitor circuit 33, 35, 35 serves to measure theamplitude and degree of modulation of the carrier.

Although the invention has been described hereinabove with reference toa particular embodiment, it will be understood, of course, that thescope thereof is not limited thereto since the various features thereofare capable of being used in other circuit ararngements and in modi fiedforms Without departing from the spirit of the present invention.

What is claimed is:

1. A tuned oscillatory circuit comprising an inductance in the forni ofa wire loop having uniformly distributed constants and a condensercoupled to the terminals of said loop, a utilization circuit coupled tosaid oscillatory circuit by means of a linear wire paralleling a portionof said loop, and means for shielding the greater part of the remainingportion of said loop.

2. A tuned oscillatory circuit comprising an inductance in the form of awire loop having uniformly distributed constants and a condenser coupledto the terminals of said loop, a utilization circuit coupled to saidoscillatory circuit by means of a linear wire paralleling a portion ofsaid loop, and means for shielding the greater part of the remainingportion of said loo-p, the length of each half of said wire loop fromlthe center thereof to said condenser being approximately one-eighth thelength of the operating wave.

3. An oscillator comprising a pair of electron discharge devicesconnected in push-pull relationship, each of said devices having ananode, cathode and a control electrode, connections between said devicesfor coupling corresponding electrodes together, said connection betweensaid anodes including a U-shaped loop, a shield at a relatively fixedradio frequency potential surrounding each leg of said U-shaped loop,and means shielding only one side of the connection extending betweenthe legs of said loop for making the capacity per unit length of saidconnection substantially the same as said legs.

4. In combination, a pair of electron discharge devices connected inpush-pull relationship, each of said devices having an anode, a cathodeand a control electrode, connections between said devices for couplingcorresponding electrodes together, the connection between said anodescomprising a tuned circuit in the form of a wire loop having uniformlydistributed constants and a condenser coupled to the terminals of saidloop, a utilization circuit coupled to said oscillatory circuit by meansof a linear wire paralleling a portion of said loop, and means forshielding the greater part of the remaining pori the legs of said loopat points removed from the connection extending between said legs, autilization circuit coupled to said tuned circuit by means of aconductor paralleling a portion of said loop, and means for shieldingthe greater part of the remaining portion of said loop.

ORVILLE E. DOW. MLS E. LINDENBLAD.

